The International Space Station, spacesuits such as the Extravehicular Mobility Unit, spacecraft, or other space structures that operate in outer space include systems that use water. Depending on the system, the water may come into contact with heat exchangers, pumps, valves, sublimators, circulation circuits in astronaut garments, and other components.
Typically the water is filtered and treated to remove contaminants and destroy micro-organisms that could otherwise foul the components and debit performance. If the filters and water treatments do not perform at expected levels or are compromised in some way, the levels of contaminants or micro-organisms may increase and, if untreated, lead to fouling. While some components are returned from space and restored at regular intervals, premature fouling may require early return, which greatly increases costs.